1. Field of the Invention
The present invention relates to a semiconductor device having a circuit constituted by electric field effect transistors (FETs) such as thin-film transistors (TFTs) and to a method of fabricating the same. TFT stands for a semiconductor element including a semiconductor layer, a gate electrode, a source electrode and a drain electrode.
In this specification, an element substrate stands for a substrate, in general. In which semiconductor elements are formed.
In this specification, a display device stands for a device, in general, which produces a bright/dark display relying upon changes in the electric signals, and a device which produces a display by applying electric signals to the liquid crystals is called a liquid crystal display device.
2. Description of the Related Art
In recent years, attention has been given to a technology for constituting TFTs by using a thin semiconductor film (several nm to several hundred nm thick) formed on a substrate having an insulating surface. The TFT is widely applied to electronic devices such as ICs and semiconductor devices, and it has been desired to develop the TFT as a switching element particularly for the liquid crystal display devices.
Known liquid crystal display devices can be roughly divided into two types: i.e., those of the active matrix type and those of the passive matrix type. A liquid crystal display device of the active matrix type uses TFTs as switching elements and is capable of displaying a high quality. The liquid crystal display device of the active matrix is usually used for the notebook-type personal computers, but it is expected that it can also be used for household TVs and portable terminals.
Among the liquid crystal display devices of the active matrix type, the liquid crystal display device of the projection type is capable of producing a display of a large size by expanding the picture on a screen. Concerning the liquid crystal display device of the projection type, technology has recently been developed to realize the portable devices by decreasing the size of the optical system by designing the liquid crystal display panel in a small size. A decrease in the size of the optical system helps lower the cost of the optical system and, hence, makes it possible to cheaply provide a liquid crystal display device.
The liquid crystal display device of the active matrix type is generally reverse-drives the lines. Among the reverse-drives of the lines, the reverse-drive of the source lines is the one in which as shown in a schematic diagram of FIG. 21, the polarity of a signal voltage written into the pixel TFTs connected to the signal lines of m columns is differed for each of the neighboring signal lines. The polarities of the signal voltages written into the pixel TFTs connected to the signal lines are changed depending upon the frames of odd numbers (FIG. 21A) and the frames of even numbers (FIG. 21B). Upon alternatingly driving the liquid crystals by changing the polarities of the signal voltages written into the pixel TFTs, the liquid crystals are prevented from being printed. The reverse-drive of gate lines is executed by replacing the signal lines of FIG. 21 by the scanning lines.
On the interface of the oriented film, the liquid crystals are so oriented as to lift up an end thereof. In this specification, a direction from an end close to the interface of the oriented film of liquid crystal molecules toward an end lifted up from the oriented film, which is orthogonally projected onto the surface of the substrate, is referred to as “pretilted direction”. Further, an angle subtended by the interface of the oriented film and by the long axis of liquid crystals near the interface of the oriented film, is referred to as “pretilted angle”. The pretilted angle is imparted by either the rubbing or the switching of liquid crystals near the interface of the oriented film by applying an electric field to the liquid crystals.
In this specification, further, the defective orientation that stems from nearly the reversed pretilted direction of the adjacent liquid crystals on the interface of the oriented film is referred to as “disclination”. Further, though the pretilted direction of the liquid crystals is the same, there exists a region where the pretilted angle locally differs due to the electric field distribution and the irregular rubbing. The defective orientation of liquid crystals that develop when the orientation is not normal turns out to be locally bright like the leakage of light when the two pieces of polarizer plates are arranged on the liquid crystal panel. The orientation of liquid crystals in which the pretilted direction is the same but in which the pretitlted angle is locally different, is referred to as “leakage of light” in this specification.
When the liquid crystal display device is driven by the active matrix system, the quality of display is spoiled by the leakage of light and disclination. That is, in the normally white mode, a light-shielding film is necessary for concealing the leakage of light and the disclination, and the numerical aperture drops.
In the liquid crystal display device in which fine pixels are formed such as the one of the projection type, the disclination and the leakage of light occur at a ratio which is no longer negligible relative to the pixels. Further, as the leakage of light and the disclination are not all concealed due to the deviation in the alignment of the light-shielding film, the leakage of light like bright line and the disclination are seen at the time of black display, and the contrast drops. That is, in the liquid crystal display device of the projection type, what is important is how to suppress the leakage of light and the disclination.
As compared to the smectic liquid crystals having a layered structure and a highly oriented order, the nematic liquid crystals tend to develop the disclination and the leakage of light due to an electric field established between a pixel electrode and another pixel electrode. In the orientation system using nematic liquid crystals, therefore, it is necessary to take a countermeasure to lower the disclination and the leakage of light.
How the leakage of light and the disclination occur will now be described with reference to FIG. 18 which is a sectional view schematically illustrating the pixel portion of the liquid crystal display device. Between the neighboring pixel electrodes in FIG. 18, it is now presumed that a first pixel electrode 101a has a potential of +5 V and a second pixel electrode 101b has a potential of −5 V. Let it now be presumed that an opposing electrode 102 has a potential of 0 V. In a region where the equipotential lines 103 are in parallel with the surface of the pixel electrode, the liquid crystals of the positive type are so oriented that the long axes of the liquid crystal molecules 108 are perpendicular to the surface of the pixel electrode. The liquid crystals of the positive type stand for the liquid crystals having a positive dielectric anisotropy. At the end of the pixel electrode, however, the equipotntial lines are bent, and the liquid crystal molecules 106 are oriented aslant with respect to the surface of the pixel electrode, i.e., are defectively oriented. It is considered that how to lower the bending of equipotential lines at the end of the pixel electrode is important from the standpoint of lowering the defective orientation.
At an end of the pixel electrode, there exists a region 104 of leakage of light where the pretilted angle locally differs. Since the equipotential lines are bent at the end of the pixel electrode, the liquid crystal molecules 106 at the end of the pixel electrode cannot be so switched that the long axes thereof become perpendicular to the surface of the pixel electrode.
Further, there exists a region where the pretilted direction of the liquid crystals becomes opposite to the pretilted direction determined by the rubbing direction 107 due to the electric field established at an end of the pixel electrode. Then, the pretilted angle and the pretilted direction locally change sharply on the interface of the oriented film, whereby the orientation of the liquid crystals is greatly distorted and the disclination occurs in the region 105.
That is, the disclination and the leakage of light are caused as the equipotential lines that are in parallel with the surface of the pixel electrode are bent at an end of the pixel electrode. In the invention described below, a structural contrivance is made so as to suppress the bending of equipotential lines as much as possible at the end of the pixel electrode.